Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (“CMOS”) image sensors (“CIS”), has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
One field of application in which size and image quality is particularly important is medical applications (e.g., endoscopes). For medical applications the chip must typically be small while providing a high quality image. In order to achieve these characteristics, for a given chip size, the photosensitive apertures should be as large as possible, while peripheral circuitry should be as limited as possible.
The pixel (picture element) fill factor denotes the fraction of the surface area of a pixel that is sensitive to light. Pixel pitch is the physical distance between adjacent pixels in an imaging device. Pixel fill factor has become smaller as pixel pitch has been reduced because the active circuit elements and metal interconnect consume an increasing proportion of the area in each pixel as the photosensitive regions are reduced in size. One way to address the loss of fill factor is to use a microscale lens (microlens) directly above each pixel to focus the light directly towards the photosensitive portion of the area within the pixel. Another way to address the loss of fill factor is to use backside illuminated (“BSI”) image sensors, which place the active pixel circuit elements and metal interconnects on a frontside of an image sensor die and the photosensitive element within the substrate facing a backside of an image sensor die. For BSI image sensors, the majority of photon absorption occurs near the backside silicon surface. However, a solution that provides larger individual pixel area on the same silicon area would improve BSI image sensors as well as frontside illuminated image sensors.